HOMOGENISING HEADS

Abstract: Described herein is an improved homogenising head in which a variable orifice can be readily provided. The head (10) comprises a casing section (12) and a plug section (14). The casing section (12) has an internally threaded portion (120) which has a series of notches (126) formed in the threads (124). The plug section (14) has an externally threaded portion (14) which has a series of flats (154) formed on the threads (152). The two threaded portions (120, 140) are engaged and the cooperation of the notches (126) and flats (154) define homogenising orifices through which the material being homogenised passes. The orifices so formed are variable in size depending on the relative positioning of the notches (126) relative to the flats (154).

Description: HOMOGENISING HEADS Field of the Invention The present invention relates to homogenisation heads and is more particularly, although not exclusively, concerned with hdmogenisation heads for producing photographic dispersions. Background of the Invention Homogenisation is the process by which very small particles are produced from an initial coarse emulsion, suspension or dispersion to give a more stable emulsion, suspension or dispersion of smaller particle size. The process takes place in a valve or head wherein the coarse emulsion, suspension or dispersion is pumped through a small gap at high pressure. Due to the high pressure, the material being homogenised has a very high velocity and as a result a large pressure drop is experienced in the valve or head. Cavitation, turbulence and shear forces act to divide the coarse material into finer particles. For example, if oil and water are to be homogenised to produce the smallest possible droplet size, a coarsely mixed liquid or 'premix 1 is passed through a high pressure homogeniser. In this process, the 'premix 1 may be pumped at pressures up to 1000 bar or even higher through a head wherein the pressure is reduced suddenly by a series of steps and narrow gaps back to atmospheric pressure. The high shear induced on the 'premix' causes the oil droplets to break up in a sequence of eddies, vortices, cavitations or impacts. It has been found empirically that when homogenising discrete phases of high viscosity, such as are found in photographic dispersions for example. then it is necessary to use an orifice type homogenising system to produce the smallest droplet size. This type of homogenising system comprises a plurality of orifice plates, each plate having at least one small hole formed therein and each plate is separated from an adjacent plate by spacers. As the 'premix' passes through the stack of plates, the dispersion is sequentially sheared. This type of homogenising system provides a finer product than systems in which the liquid is forced between plates held together by an adjustable spring. GB-A-1 577 567 describes such a system. A high pressure 'premix' is directed through a multi¬ stage device in which, at each stage, the material is directed through a nozzle at high speed and then impinges on a wall of an orifice block perpendicular to the direction of flow. The first break up and ' dispersion of the 'premix 1 occurs due to the shearing force exerted on the 'premix' as it passes through the nozzle. On impingement on the wall, the direction of flow of the 'premix' at high pressure is rapidly changed and a second break up and dispersion occurs. The material then flows through axially bored holes offset from the nozzle formed in the orifice block on to the next stage. A high degree of dispersion is obtained by the multi-stage effect. However, with a fixed orifice homogenising system, the pressure and flow are inseparably linked and one cannot be adjusted independently of the other. This is normally acceptable if the material being homogenised is made in a batch-wise fashion where the batch can be made under any reasonable pressure/flow conditions and stored prior to use. DE-A-3 728 946 discloses a homogenising head in which a piston-valve arrangement is used. Liquid to be homogenised is passed through an inlet formed in the valve seat at high pressure through a homogenising channel defined by a piston and a valve seat each having a diamond-sintered layer. The valve has a surface which is angle at 5° to the end surface of the piston. Homogenising material is then collected from a collection chamber through an outlet. US-A-4 348 116 describes a homogenising head arrangement in which a casing is provided with an internal cylindrical surface defining a bore, a portion of internal surface being threaded. The casing has an open end into which a rotatable closing plug is inserted, a portion of the outer surface of the plug being threaded for engagement with the threaded portion of the internal surface of the casing. The casing also includes an inlet port through which the liquid to be homogenised is introduced and an outlet port through which the homogenised liquid can be collected. Longitudinally extending grooves or slots are provided along the threaded portion of the plug and extend to a depth which is more than the depth of the thread formed in the threaded portion, and cuts into the plug forming a continuous trench. These grooves or slots, which together with the threaded portions of the casing and the plug, define homogenising passages through which liquid is directed under pressure from the inlet port. The homogenised liquid produced after passing through these homogenising passages is then directed to the outlet port. The bore in the casing has a first longitudinal axis, and the threaded portion is truncated in transverse section in a progressively variable fashion by providing a second bore having a second longitudinal axis which is displaced vertically from the first longitudinal axis. This means that the threaded portion of the internal cylindrical surface of the casing has threads which have an eccentric height on progressing a full turn around the internal surface of the casing. Rotation of the plug relative to the casing controls the variation of the section of the homogenising passages. In particular, as the plug is rotated inside the casing through 180°, the edge of the threads of the thread inside the casing close in on the bottom of the trench formed in the plug to close the sequence of gaps between successive tetrahedral chambers so formed. Problem to be solved by the Invention However, there are problems associated with the reproducibility of setting up this homogenising head for operation. In particular, it is difficult to set it up to operate at particular high pressures. Furthermore, fluctuations in the flow rate may be obtained at settings at which the best dispersions would be expected. The system described in GB-A-1 577 567 has the disadvantage that there is an inseparable link between pressure and flow as discussed above. Furthermore, such a system cannot be conveniently adjusted to identify an optimum orifice size or number of orifices for a given product. Moreover, no adjustment is possible to allow for small changes due to wear. The device described in US-A-4 348 116 has unsupported thread portions due to the eccentricity of the second bore and these threads may be deformed when operating at the highest pressures. Moreover, a very accurate double boring operation is required in the manufacture of the casing to produce the truncated thread portions thus limiting the options for variation in geometry of the device. Summary of the Invention It is therefore an object of the present invention to provide an improved homogenising head which is simpler to manufacture and overcomes the problems associated with fluctuating flow rates and reproducibility of set up. It is a further object of the present invention to provide a homogenising head in which independent control of pressure and flow can be achieved. In accordance with one aspect of the present invention, there is provided a homogenising head comprising:- a casing portion having at least one open end, an inlet port, an outlet port and at least one internally threaded portion situated between the inlet port and the outlet port; a plug portion having at least one externally threaded portion for engaging the at least one internally threaded portion of the casing portion as the plug portion is inserted into the casing portion; and channel defining means for defining at least one homogenising channel across the engaged threaded portions of the casing portion and the plug portion; characterized in that the channel defining means comprises at least a first region formed in the internally threaded portion of the casing* portion and at least a second region formed on the externally threaded portion of the plug portion, each first and second region cooperating with one another in dependence on their relative position to vary continuously the size of the associated homogenising channel defined thereby between a fully open position and a fully closed position. Advantageous Effect of the Invention By this arrangement, a simpler homogenising head is provided in which a given set of conditions can be repeatedly achieved. In particular, the relative position of each first and second region provides independent control of pressure and flow. The homogenising head according to the present invention is made by simple single-axis boring and threading. As a consequence, it is easier to construct that the prior art device described in US-A-4 348 116. Moreover, there is no unnecessary limitation of the options to vary the geometry of the homogenising channels. Brief Description of the Drawings For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawings in which:- Figure 1 is a schematic cross-section through one embodiment of a homogenising head constructed in accordance with the present invention on an exaggerated scale; Figure 2 is a schematic sectioned perspective view illustrating the cooperating flats and notches of in the homogenising head shown in Figure 1; Figure 3 is a more detailed view illustrating orifice size as defined by the cooperation of the notches and slots; Figure 4 illustrates a partial cross-section through a plug portion in accordance with the present invention; Figure 5 is a sectioned view taken on lines V-V in Figure 4; Figure 6 is a sectioned view taken on lines VI-VI in Figure 4; Figure 7 illustrates a part of a casing portion in accordance with the present invention; Figure 8 is a sectioned view taken on lines VIII-VIII in Figure 7; Figures 9 to 12 illustrate respective enlarged and exaggerated cross-sections through the homogenising head in accordance with the present invention showing the notch of the casing portion aligned with different regions of the flat of the plug portion; Figure 13 illustrates a plug section having flats which extend around half its circumference; Figures 14 to 17 illustrate notches of different cross-section; Figure 18 illustrates a schematic cross- section through a second embodiment of a homogenising head constructed in accordance with the present invention; Figure 19 is a graph showing water calibration of the homogenising head of the present invention and illustrating the effect of pressure and angle on flow; Figure 20 is a graph showing contours of reactivity for a photographic dispersion at given pressures and angles; Figure 21 is a graph illustrating the pressure/flow relationship for a photographic dispersion for a range of angles in the device according to the present invention; Figure 22 is a graph illustrating the effect of changing the number of engaged threads on reactivity and droplet size for a photographic dispersion for a device according to the present invention; Figures 23 is a graph illustrating the effect of pressure on reactivity and droplet size for a device according to the present invention wherein four threads are engaged; Figures 24 and 25 are graphs illustrating the effect of homogenising device angle and pressure on reactivity and droplet size respectively; and Figures 26 and 27 are graphs illustrating the effect of pressure on droplet size and reactivity respectively for four different homogenising heads. Detailed Description of the Invention Figure 1 illustrates one embodiment of a homogenising head 10 constructed in accordance with the present invention. The head 10 comprises a casing section 12 and a plug section 14. The casing section 12 has an internally threaded portion 120 and a housing portion 122. An inlet 16 is provided in the casing section 12 as shown. The plug section 14 has an externally threaded portion 140, which is sized for engagement with the internally threaded portion 120 of the casing section 12, and a shaft portion 142. Two annular grooves 144, 146 are formed in the shaft portion 142 in which O-ring seals 18, 20 are arranged to seal against the housing portion 122 as shown. Naturally, a different number of grooves may be provided to accommodate different numbers of O-ring seals according to the particular application. This seal system acts to close off the flow of liquid towards end 22 of the head 10. End 148 of the plug section 14 carries a bar portion 150 by means of which the plug section 14 can be rotatably inserted into the casing section 12 and then turned to alter the relative position of the plug section with respect to the casing section once inserted. The head 10 also includes a collection chamber 24 having an outlet 26. The threads of the internally threaded portion 120 (female threads) and the externally threaded portion 140 (male threads) are axially symmetric. Each thread 124 of the internally threaded portion 120 of the casing section 12 has a notch 126 formed in it. Similarly, each thread 152 of the externally threaded portion 140 of the plug section 14 has a flat 154 formed on it. Each notch 126 and flat 154 is not cut to the bottom of its respective thread 124, 152. This enables a variable area orifice to be produced by changing the relative positions of the notches 126 and flats 154. The orifice defines a homogenising channel through which material to be homogenised passes during the homogenisation process. Furthermore, as the notches 126 and flats 154 are cut only into a part of the depth of their respective threads 124, 152, in all relative orientations of the plug section 14 and the casing section 12, the male and female threads interlock securely over a substantial fraction of the depth of the threads, and liquid can flow through the well supported cavities so formed under high pressure. In operation, a 'premix', that is, a coarse mixture of the components to be homogenised is directed at high pressure into the head 10 at its inlet 16. The 'premix' then passes through the homogenising channels defined by the orifices formed by notches 126 and flats 154 in the engaged threaded portions 120, 140 to the collection chamber 24. Homogenising of the 'premix' is achieved by the material passing through one orifice and into a larger chamber defined by the flat 154 in the plug section 14 and the associated threads 120 of the casing section 12. From there, the material then passes to the next orifice and larger chamber etc. The homogenised material can then be removed from the chamber 24 through the outlet 26. Figure 2 shows schematically the relative positioning of the notches 126 and flats 154 in more detail. Each notch 126 is a fixed size and cooperates with a flat 154 which varies in depth along its length. When a notch 126 lies over a flat 154, an orifice is formed through which the 'premix' is directed at high pressure. The size of this orifice can be varied in accordance with the relative position of the notch 126 with respect to the flat 154, that is, the position of the notch 126 along the length of the flat 154. In Figure 3, various positions of a notch 126 are shown relative to a flat 154. As shown, at 0°, the orifice will have its greatest size as a major part of the notch 126 overlies the lowest part of the flat 154. As the relative angle is increased, the orifice size decreases due to the notch 126 overlying higher parts of the flat 154. In this particular arrangement, the flat 154 varies in height over an angle of 90° and as a result, at a relative angle of 90°, the orifice is closed off entirely. However, other arrangements are possible where smaller or larger relative angles may be provided over which the orifice size can vary. Referring now to Figures 4 to 6, sections through a thread 152 are shown. In Figure 4, plug section 14 is shown with a flat 154 formed on a thread 152. Figures 5 and 6 illustrate sections through the thread 152 at the positions shown by lines V-V and VI- VI respectively. Similarly, in Figures 7 and 8, sections through a thread 124 are shown. Figure 7 shows casing section 12 with a notch 126 formed on a thread 124. Figure 8 illustrates a section through the thread 124 at the line VIII-VIII. Figures 9 to 12 are sections through the head 10 with notch 126 aligned along flat 154 at different relative positions. Dotted line 30 marks the position of the notch 126 in each case. In Figure 9, the angle between the notch 126 and the flat 154 is 0°. Here, the orifice is fully opened as described previously. As the flat 154 is moved relative to the notch 126 to an angle of 30° (as shown in Figure 10) , the orifice becomes smaller. At the 60° position, shown in Figure 11, the orifice is smaller still and is finally closed in the 90° position shown in Figure 12. In the embodiment of the plug section 14 described above, five threads 152 have been shown with flats 154 extending around a quarter of the circumference (90°) of each thread 152. Naturally, different lengths can be given to the flat 154 depending on the particular application. Figure 13 illustrates a part of a plug section 14 in which the flats 154 extend around half the circumference of the thread 152, that is, through an angle 180°. In this case, the orifice would be open at the 0° position as before, but would not be closed off until the 180° position. Further extension of the flats to almost the entire circumference of the thread may be possible to provide finer control over a wider range of angles. It will be readily appreciated that there may be more or less than five threads formed on the plug section 14 according to the particular homogenising application. Furthermore, not all the threads need be engaged. This has the effect of cutting down the number of orifices through which the material being homogenised passes during the homogenising process. As shown in Figures 14 to 17, the notch 126 may have different cross-sections. The notch 126 need not be triangular as described above and may have a rectangular cross-section as shown in Figure 14 or a square cross-section as shown in Figure 15. Moreover, the notch 126 may vary in cross-section over several threads from a size shown in Figure 16 to that shown in Figure 17 and vice versa. This would have the effect of having progressively sized orifices. In a second embodiment of a homogenising head 10' in accordance with the present invention, a second internally threaded portion 120' may be provided in casing section 12', and a second externally threaded portion 140' provided in plug section 14'. These two portions 120', 140' are not provided with notches and flats in a similar way to portions 120, 140, but are present to balance the pressures applied to the head 10'. Portions 120', 140' carry out a similar function to the '0' ring seals described with reference to Figure 1. Threaded portions 120', 140' may be cut to a different diameter but have identical pitches to threaded portions 120, 140. Portion 140' is connected to portion 140 by a 'waisted' or shaft portion 40. This head 10' operates as described above. It will be appreciated that the flats and notches described above may be interchanged and provide the same effect, that is, the notches may be cut in the plug section and the flats in the casing section. Although the present invention has been described with reference to embodiments in which a single flat/notch is provided on each thread, it may be the case where two or more evenly spaced flats/notches are utilised. Flow of the 'premix' through the head may be substantially increased. The flats may have any suitable profile. However, the profile described above, that is, the depth changing along the length, has the advantage that positioning is not duplicated and that there is opportunity for extending adjustments over a greater arc than would be the case if the flat were not varying in height around the circumference of the thread 152. The notches and flats described above are axially aligned with the longitudinal axes of both the casing section 12 and the axis of the plug section 14. It may be advantageous to align the notches and/or flats so that they are skewed with respect to the longitudinal axes providing a more tortuous path for the material as it passes through the homogenising head. It may also be advantageous to modify the notch/flat arrangement to incorporate a feedback system to compensate for fluctuations in pressure of the 'premix' . This has the effect of maintaining a more constant product, that is, constant droplet size and/or reactivity. For example, the feedback system may be provided by angling the notches relative to the flats such that the 'premix' passing through the notch impinges on a portion of the flat to move the casing section 12 relative to the plug section 14 against an opposing torque. Heads according to the present invention may be made from stainless steel which has high corrosion resistance and is relatively easy to clean. Contamination of the product is also reduced. However, if the head is to be used in situations where the casing section and the plug section are to be forced together under high pressure (stainless steel having a tendency to bind under these conditions) , it may be necessary to plate at least one of the two sections to improve the slip of one surface over the other thereby preventing binding. An example of such plating would be electroless nickel plating. For example, the plug section could be made very slightly undersize and then a very thin layer of* nickel plated on to its surface. This is advantageous as nickel is a hard metal and has good corrosion resistance. Heads according to the present invention may be made from materials other than stainless steel. For example, ceramic materials can be used for either one or both sections of the head. Furthermore, other surface treatments may also be used, for example, hardening and diamond coating. A homogenising arrangement was constructed using a head in accordance with the present invention as described above. The head was attached homogenising apparatus sold by APV Rannie AS of Albertslund, Denmark (Lab type 16-51H) , and experiments were carried out. Experiment 1. In this experiment, pressure/flow relationship through the head was to be determined. Water was used as the liquid for simplicity. The position of the flats with respect to the notch was varied over an angle of about 90°. The pressure was varied over about 150 bar (15MPa) . A range of pressure/flow/angle values were obtained and converted to a contour plot of flow with pressure on one axis and angle on the other. The plot obtained is shown in Figure 19. From this, it can be seen that with a constant pressure, it is possible to obtain a flow varying over about 1.51/min for an angle variation of about 90°. Experiment 2. In this experiment, a photographic dispersion was used. A premix of an Ektacolor (Registered Trade Mark) dispersion with about 8% gelatin was passed through the head using a range of pressures, flows -and different angles. It was found that a wide range of reactivities could be obtained for a range of angles and pressures. The results obtained from this experiment are given in Tables 1 and 2 below. Table 1, Table 2 screw anσle at 300 bar (°) reactivity 90 553 60 848 extrapolation to 30 1143 (Screw angle is the relative angle between the plug section and casing section as described with reference to Figures 9 to 12 above.) It was noted that the reactivity increased as the orifice was opened. This implies that there may be an optimum size of orifice for a given system, and it can also be seen that the droplet size decreases as the pressure is increased. This is also shown graphically in Figures 20 and 24 described below. Using other collected during the trials, a contour plot relating relative angle and pressure was obtained. This plot is shown in Figure 20. It was apparent that with modest pressures and smaller angles, an appropriate reactivity level could be attained. Figure 21 shows a plot of pressure against flow for particular screw angles. The solid and dashed lines respectively indicate the results obtained for a four-thread engagement and for a six- thread engagement. A comparison with a single orifice system having five plates is shown by the dotted line. Figure 22 shows the effect of the number of engaged threads on reactivity and droplet size. As shown, by increasing the number of threads engaged from 3 to 6, the reactivity increases by approximately 100 units. Similarly, droplet size decreases by 0.02μm as the number of threads engaged increases from 3 to 6. Figure 23 shows the effect of homogenising head angle and pressure on reactivity and droplet size for a device having four engaged threads. The screw angle for the device according to the present invention was 0°. Figures 24 and 25 are plots which illustrate the effect of homogenising head angle and pressure on reactivity and droplet size respectively. As shown in Figures 24 and 25, it is clear that better reactivity values and smaller droplet sizes are obtained with high pressures and low screw angles. It is also to be noted that there is much less change in reactivity and droplet size with small pressure changes at small screw angles. This may define a robust region compared to the small orifice region (screw angles around 90°) where similar small changes in pressure result in large changes in reactivity and droplet size respectively. Figures 26 and 27 show the comparison between two spring-loaded valve heads (shown as solid (No. 1) and dotted (No. 2) lines respectively), fixed orifice plates (shown as a dashed line) and the device according to the present invention (shown as a dot- dashed line) for the effect of pressure on droplet size and reactivity respectively. As shown in the Figures illustrating the test results obtained with the device of the present invention, the properties of the dispersion, namely, reactivity and droplet size, can be controlled by the number of orifices through which the material passes during homogenisation, the homogenisation pressure, and the size of the orifice or screw angle. It is to be noted that the present invention can be used for other dispersions and is not limited to use with photographic dispersions. Naturally, the dimensions and configuration of the homogenising head are chosen in accordance with the particular application for which it is to be used.

Claims: CLAIMS : 1. A homogenising head comprising:- a casing portion having at least one open end, an inlet port, an outlet port and at least one internally threaded portion situated between the inlet port and the outlet port; a plug portion having at least one externally threaded portion for engaging the at least one internally threaded portion of the casing portion as the plug portion is inserted into the casing portion; and channel defining means for defining at least one homogenising channel across the engaged threaded portions of the casing portion and the plug portion; characterized in that the channel defining means comprises at least a first region formed in the internally threaded portion of the casing portion and at least a second region formed on the externally threaded portion of the plug portion, each first and second region cooperating with one another in dependence on their relative position to vary continuously the size of the associated homogenising channel defined thereby between a fully open position and a fully closed position. 2. A head according to claim 1, wherein one of the first or second regions comprises at least one notch portion formed on respective ones of the internally threaded portion of the casing portion or the externally threaded portion of the plug portion. 3. A head according to claim 2, wherein each notch portion is substantially aligned with the longitudinal axis of the threaded portion on which it is formed. 4. A head according to claim 2, wherein each notch portion is skewed with respect to the longitudinal axis of the threaded portion on which it is formed. 5. A head according to any one of claims 2 to 4, wherein a plurality of notch portions are provided, each notch portion increasing size along the length of the threaded portion on which it is formed. 6. A head according to any one of claims 2 to 4, wherein a plurality of notch portions are provided, each notch portion decreasing size along the length of the threaded portion on which it is formed. 7. A head according to any one of claims 2 to 6, wherein each notch portion is cut to a depth which is less than the full depth of the threaded portion on which it is formed. 8. A head according to any one of claims 2 to 7, wherein the other one of the first or second regions comprises at least one flat portion formed on respective ones of the internally threaded portion of the casing portion or the externally threaded portion of the plug portion, alignment of one flat portion with one notch portion defining a homogenising channel. 9. A head according to claim 8, wherein each flat portion is substantially aligned with the longitudinal axis of the threaded portion on which it is formed. 10. A head according to claim 8, wherein each flat portion is skewed with respect to the longitudinal axis of the threaded portion on which it is formed. 11. A head according to any one of claims 8 to 10, wherein a plurality of flat portions are provided, each flat portion increasing size along the length of the threaded portion on which it is formed. 12. A head according to any one of claims 8 to 10, wherein a plurality of flat portions are provided, each flat portion decreasing size along the length of the threaded portion on which it is formed. 13. A head according to any one of claims 8 to 12, wherein each flat portion is cut to a depth which is less than the full depth of the threaded portion on which it is formed. 14. A head according to any one of claims 8 to 13, wherein each flat portion varies in height along its length around the circumference of the threaded portion on which it is formed.

Inventors: YOUNG DAVID JOHN, YOUNG, DAVID, JOHN
Applicants: EASTMAN KODAK CO, KODAK LTD, YOUNG DAVID JOHN, EASTMAN KODAK COMPANY, KODAK LIMITED, YOUNG, DAVID, JOHN
Application year: 1994
Main classification: B01F5
IPC codes: B01F3, B01F5